1. Field of the Invention
This invention relates to a photovoltaic device that converts incident light energy to electrical energy, and more specifically it relates to a photovoltaic device that is suitable for use as a thin-film solar cell.
2. Description of the Prior Art
Silicon solar cells, which are a type of photovoltaic device, for power applications include mainly those cells that utilize a single-crystalline silicon or polycrystalline silicon substrate about 300 xcexcm thick, but thin-film substrates are desired to reduce the amount of silicon used and reduce cost.
A number of measures have been taken to improve the characteristics of thin-film solar cells. In order to improve conversion efficiency, for example, a structure was proposed for the purpose of improving conversion efficiency, wherein the glass substrate is provided with a textured reflective mirror, whereon a textured silicon layer is deposited (K. Yamamoto, IEEE Trans. ED, pp. 2162-2164, 1999). A structure has also been proposed, wherein after a transparent substrate is adhered to the surface of a single-crystalline thin-film solar cell formed on a single-crystalline silicon substrate with a porous silicon layer between them, the thin-film solar cell is separated from the single-crystalline silicon substrate and is adhered to a different substrate (JP-A HEI 10-150211).
In the thin-film solar cells described above, which are photovoltaic devices of the prior art, even though the light-receiving surface and the opposite surface are relatively dose and this characteristic affects power generation efficiency, no consideration has been made to remove that effect by making the semiconductor surface opposite the light-receiving surface inactive. Therefore, the surface recombination velocity of carriers in the semiconductor surface opposite the light-receiving surface is high, which is a factor in lowering the open-circuit voltage. The effect of the surface recombination velocity on the power generation characteristic is particularly great in thinner solar cells, and therefore passivation at the semiconductor substrate surface is an important factor in improving conversion efficiency.
This invention is proposed to address the above problem, and an object thereof is to offer a photovoltaic device that improves conversion efficiency regardless of whether the light-receiving surface and the opposite surface are in dose proximity by passivating the semiconductor surface opposite the light-receiving surface of the photovoltaic device.
In order to achieve the above object, the photovoltaic device of this invention, which converts incident light energy to electrical energy, comprises a single-crystalline or polycrystailine semiconductor substrate, an n-type diffusion layer region and a p-type diffusion layer region formed adjacent to each other on one surface of the semiconductor substrate, a first electrode electrically connected to the n-type diffusion layer region, a second electrode electrically connected to the p-type diffusion layer region, an adhesive layer formed on the other surface of the semiconductor substrate and containing an inorganic binder and a filler, and a supporting substrate adhered to the adhesive layer.
Further, the photovoltaic device of this invention comprises a single-crystalline or polycrystalline semiconductor substrate, an n-type diffusion layer region formed on one surface of the semiconductor substrate, a first electrode electrically connected to the n-type diffusion layer region, a p-type diffusion layer region formed on the other surface of the semiconductor substrate, a second electrode electrically connected to the p-type diffusion layer region, an adhesive layer formed on the other surface of the semiconductor substrate and containing an inorganic binder and a filler, and a supporting substrate adhered to the adhesive layer.
The inorganic binder contained in the adhesive layer may be an alkali silicate (sodium silicate, potassium silicate, or lithium silicate) or a metal phosphate (aluminum phosphate or magnesium phosphate).
The index of refraction of the adhesive layer may be not less than 1.4 and not more than 3.5.
The filler contained in the adhesive layer may be an aluminum oxide, titanium oxide, or barium oxide.
The photovoltaic device of this invention, as is dearly described above, uses an adhesive, which contains a substance that passivates the semiconductor surface, to adhere a diffuse-reflection substrate, which diffuses light it reflects, to the surface opposite the light-receiving surface of the semiconductor substrate, whereby the semiconductor substrate surface is passivated, and as a result the conversion efficiency is improved. In particular, a photovoltaic device with this structure can be used effectively in a thin-film solar cell wherein the thickness of the semiconductor layer is from 1 xcexcm to 50 xcexcm.
The above and other objects and features of this invention will be described below in detail based on the accompanying drawings.